This invention relates to methods and systems capable of efficiently drilling offshore wells in extremely deep water using a smaller, more economical floating vessel, along with smaller, and less expensive, drilling equipment (such as hoisting equipment, riser tensioners, mud systems, etc.) than heretofore possible. This is possible because the system is able to perform all requisite tasks and functions using a reduced diameter marine riser that dramatically reduces variable deck load and space requirements for the vessel.
In recent years, the search for oil and gas deposits has taken oil companies into ever deeper offshore waters. Floating rigs of only a few years ago were generally limited to perhaps 1,500 feet of water depth, but it is now commonplace to conduct offshore drilling operations in water depths up to 5,000 feet, and several rigs are under construction which are theoretically capable of conducting drilling operations in 10,000 feet of water or more. For extreme water depths, dynamic positioning, which is not sensitive to water depth, is commonly used for vessel station keeping.
The basic deep water drilling system is unchanged from that designed more than twenty years ago. The system employed to actually drill a well in deep water is basically an extension of that for drilling in shallower water. Typically, this system employs subsea components consisting of an 18xc2xexe2x80x3 subsea blowout preventer (BOP) stack installed at the ocean floor and coupled to a floating drilling rig at the ocean surface by a 21xe2x80x3 diameter marine riser system. This arrangement allows the driller to utilize the riser to convey to, and install, the typical 18xc2xexe2x80x3 API subsea BOP stack on the wellhead, and supports a well program typically including 30xe2x80x3, 20xe2x80x3, 13xe2x85x9cxe2x80x3, 9⅝xe2x80x3, and 7xe2x80x3 casing. Occasionally, additional strings of casing and/or liner may be employed.
The major adaptation of the riser system for deeper water has been to lengthen it. Lengthening the riser requires greater material strength, thicker walls, additional and larger service lines, more exotic riser connectors and tensioner system, and thicker and denser floatation. Unfortunately, lengthening the marine riser gives rise to significant consequential rig related issues as well, which, as will be shortly disclosed, tend to dominate deep water rig design, particularly semisubmersible rig design.
All marine risers must be maintained in tension whenever they are deployed; the minimum tension requirement is that the riser not be in compression at the top of the subsea BOPs. The weight of riser which the tensioning system must support is comprised of two main elements. The first is the steel weight of the riser tubing, joining connectors, auxiliary conduits, and control lines. Syntactic foam buoyancy modules are strapped around the riser to compensate for part of the riser steel weight when the riser is in the water, but these modules add to the weight in air and increase the overall diameter of the riser to around 56xe2x80x3. By way of example, the weight in air of 10,000 feet of a 21xe2x80x3 marine riser with buoyancy modules is approximately 3,600 tons.
In addition to the steel weight, the tensioning system must provide sufficient axial tension at the top of the riser to control the stresses and displacement of the riser while the floating drilling vessel moves horizontally and vertically in response to wind, waves and current. The tension requirements increase with increasing drilling mud weights and riser offsets. This means that even after considering the buoyancy, the riser tensioners for 10,000 feet of water have a total tensioning capacity of about 1,550 tons. In addition, while the actual drilling operation only requires about 500 tons of hoisting capacity, this must be increased to 750 to 1,000 tons to handle the riser and BOPs in deep water.
The riser required for 1,500 feet of water weighed only about 150 tons in air, did not normally require much buoyancy, and could be stowed in about 1,200 square feet of deck space. The marine riser for 10,000 feet of water weighs about 3,600 tons in air and requires a storage area of about 10,500 square feet.
The marine riser is subjected to lateral forces due to ocean currents, and these forces are proportional to the riser diameter. The lateral forces are transmitted to the vessel at the surface, and ultimately must be resisted by the vessel""s station keeping system. Current flow around the riser also results in vortices, which, when shed, xe2x80x9cpluckxe2x80x9d the riser and induce low frequency oscillations in the riser, causing stress and fatigue. The riser for 1,500 feet of water had an effective diameter of about 36xe2x80x3, while that for 10,000 feet of water has an effective diameter of about 56xe2x80x3 due mainly to the use of syntactic foam buoyancy modules. Consequently, a deep water riser is subjected to greater lateral forces and stresses than a riser designed for use in shallower water.
It is sometimes required to disconnect the riser from the blowout preventers during the course of a well to effect repairs to subsea components, or in an emergency occasioned by a station keeping failure. Prior to any planned riser disconnect, the mud in the riser is displaced with seawater with the mud being returned to the mud pits on the vessel. The mud to be displaced, and stored on the vessel, that is contained in the marine riser in 1,500 feet of water, is about 600 bbls and weighs about 200 tons. Conversely, 10,000 feet of riser contains about 3,600 bbls of mud weighing nearly 1,200 tons.
In deeper sections of an offshore well where the hole-drilling diameter is small, the rate of mud circulated through the bit is reduced proportionately. For these sections, the annular velocity of the mud returns in the 21xe2x80x3 marine riser is quite low, and while this is not much of a problem with shorter risers, in deeper water it is insufficient in the riser to carry drilled cutting solids to the surface, and an additional mud pump is required to circulate or xe2x80x9cboostxe2x80x9d the marine riser.
The overall cost of a deep water drilling unit is proportional to its displacement size, variable load requirements, and equipment capacity. By way of example, a conventional design for a shallow water drilling unit and a deep water drilling unit may have the following capabilities and costs:
The increased size and cost of a deep water drilling unit are directly related to the increased length of the riser. It is postulated that the size and cost of a deep water rig will, within certain limits, be approximately proportional to the square of the riser diameter, and that if the riser diameter could be reduced to about ⅔ of its present diameter, the size and cost of a rig might be reduced by 40 percent or more.
The present invention is directed to a fully capable and functional drilling system capable of drilling, and/or, working over wells presently requiring the use of a 21xe2x80x3 marine riser while utilizing a reduced diameter riser. By way of example, the present invention may use a riser having a nominal diameter of about 15xe2x80x3. Consequently, use of the present invention will reduce the variable deck load, space requirements, hoisting, mud pit and pump capacities and, hence, the cost of a deep water floating drilling vessel.
The present invention is directed to a deep ocean drilling system for drilling an offshore well in deep water using a reduced diameter drilling riser. The reduced diameter drilling riser extends from a floating drilling vessel, such as a drill ship or a semisubmersible drilling rig, to a lower marine riser package. The lower marine riser package includes a lower marine riser package connector, a riser flex joint and possibly an annular blowout preventer. The drilling system also comprises a retrievable high pressure blowout preventer stack attached to a high pressure wellhead housing. The blowout preventer stack usually includes one or more annular preventers, one or more ram preventers, and a lower marine riser package mandrel, whereby the lower marine riser package connector may be releasably connected to the blowout preventer stack. A fluid diverter line extends from the blowout preventer stack to a fluid return mandrel, whereby the lower marine riser package connector may be releasably connected to the fluid return mandrel. Thus, the fluid return mandrel serves as an alternative, or secondary, riser support station on the blowout preventer stack. The drilling system also includes a retrievable lifting and guide frame assembly comprising an upper lifting frame and a lower guide frame. The lifting frame is connected to the lower marine riser package connector. The lower marine riser package connector and the upper lifting frame are vertically and laterally moveable within a slot formed in the lower guide frame to maintain the axial alignment of the riser and provide a pathway for controlled movement of the riser between the lower marine package mandrel and the fluid return mandrel.
The drilling system further comprises choke and kill lines, hydraulic power and control lines extending from the drilling vessel and releasably connected to the blowout preventer stack, wherein such lines remain functional and protected from mechanical damage when the lower marine riser package connector is disconnected from the lower marine riser mandrel and reconnected to the mud return mandrel. Likewise, the choke and kill lines, hydraulic power and control lines remain functional and protected from mechanical damage when the lower marine riser package is disconnected from the mud return mandrel and reconnected to the lower marine riser mandrel.
In another embodiment of the present invention, the blowout preventer stack, diverter line and fluid return mandrel are self-contained within a support frame. The lower guide frame may be releasably connected to the blowout preventer support frame. The choke and kill lines, hydraulic power and control lines are also releasably connected to the blowout preventer stack. The blowout preventer stack and the lower marine riser package each contain receptacles for the control pod and choke and kill lines.
The mud diverter line of another embodiment of the present invention includes a riser dump valve to allow well flow to be diverted to the sea at the wellhead, or to dump heavy mud from the riser without disconnecting the riser from the blowout preventer stack. The riser dump valve also allows the well to fill with seawater. The mud diverter line provides a means to independently circulate the well and the marine riser and to displace either to sea water or other fluid, such as drilling mud, while the riser is connected to the secondary riser support station. The blowout preventer stack may further include a rotating head for sealing about the drill string when the riser is connected to the fluid return mandrel. Such an arrangement would permit drilling while the riser is connected to the mud return mandrel whereby mud circulated to the drilling bit would be diverted to the riser and returned to the drilling vessel.
In another embodiment of the invention, fairings are included on all riser connection flanges and the lower marine riser package to deflect equipment being lowered in open water away from the riser to minimize or eliminate damage resulting from possible collisions.
In another embodiment of the invention, a deep ocean drilling system for drilling offshore wells from a drilling vessel includes a lifting and guide frame assembly comprising an upper lifting frame connected to the lower marine riser package connector and a lower guide frame connected to the blowout preventer stack, wherein the lifting frame restricts the vertical movement of the lower marine riser package connector and the guide frame restricts the lateral movement of the lower marine riser package connector to maintain the axial alignment of the riser and control the movement of the riser between the lower marine riser package mandrel and the secondary support mandrel. The deep ocean drilling system may comprise a guide funnel attached to the lifting frame and positioned directly above the blowout preventer stack when the lower marine riser package connector is connected to the secondary support mandrel.
In another aspect of the invention, a riser system for connecting a subsea blowout preventer stack to an offshore drilling vessel is provided which comprises a riser pipe extending from the drilling vessel to a lower marine riser package connector; a blowout preventer stack having a lower marine riser package mandrel wherein the lower marine riser package connector may be releasably connected to the lower marine riser package mandrel; a secondary support mandrel wherein the lower marine riser package connector may be releasably connected to the secondary support mandrel; and a guide frame assembly comprising a guide frame attached to the blowout preventer stack, a guide pin attached to the lower marine riser package connector, the guide pin retained within a slot formed in the guide plate, the guide plate being attached to the guide frame by one or more pivotable arms wherein the slot in the guide plate restricts the vertical movement of the lower marine riser package connector relative to the blowout preventer stack and the arms restrict the lateral movement of the lower marine riser package connector between the lower marine riser package mandrel and the secondary support mandrel to maintain the axial alignment of the riser during movement of the riser between the lower marine riser package mandrel and the secondary support mandrel. The guide frame assembly may comprise a hydraulic actuating arm attached to the guide frame at one end and attached to the arms wherein the ram can be actuated to laterally move the lower marine riser package connector from the lower marine riser package mandrel to the secondary support mandrel, or vice versa.
In another aspect of the invention, a subsea wellhead system is provided comprising a wellhead housing having an internal bore with a landing means in the bore, a casing hanger having an external shoulder for landing on the landing means of the wellhead housing, wherein the casing hanger has an internal bore with an internal landing means for supporting subsequent casing strings. By way of example, the subsea wellhead system may comprise a 18xc2xexe2x80x3 wellhead housing and a 13xe2x85x9cxe2x80x3 casing hanger with an internal bore configured with an internal landing means for supporting subsequent casing strings. The subsequent casing strings may include a 9⅝xe2x80x3 casing string with a 9⅝xe2x80x3 casing hanger and a 7xe2x80x3 casing string with a 7xe2x80x3 casing hanger and a suitable tubing hanger.
The present invention also pertains to a method of drilling a well in deep water from a floating drilling vessel having a reduced diameter riser for connecting the vessel to the well. The method comprises the steps of providing a lower marine riser package on the end of the reduced diameter riser to connect the riser to a lower marine riser mandrel on a high pressure blowout preventer stack, disconnecting the lower marine riser package connector on the lower marine riser package from the lower marine riser mandrel, repositioning the riser over a secondary riser support mandrel on the blowout preventer stack, connecting the lower marine riser package connector to the secondary riser support mandrel, wherein a fluid diverter line provides fluid communication between the secondary support mandrel and the blowout preventer stack, and lowering a 13xe2x85x9cxe2x80x3 casing string outside of the riser through the blowout preventer stack and into the well while the well is in fluid communication with the riser. The method further comprises installing an automatic casing fill-up float shoe on the casing to minimize casing float and, thus, buckling forces due to a lack of lateral support of the casing string from the marine riser. Alternatively, the 13xe2x85x9cxe2x80x3 casing string may be run open ended without float equipment to minimize casing float and buckling forces.
The method may further comprise the step of using active motion compensation to affect disconnection, reconnection, and stabbing operations. An auxiliary hoist may be used to lower the 13xe2x85x9cxe2x80x3 casing to the blowout preventer stack and into the well.
The method of the present invention may include stripping a 13xe2x85x9cxe2x80x3 casing string through the blowout preventer stack while taking returns through the riser as the casing string is lowered into the well.
The method may further comprise providing the 13xe2x85x9cxe2x80x3 casing string with a 13xe2x85x9cxe2x80x3 casing hanger and landing the hanger in a subsea wellhead housing, the casing hanger having an internal bore with a landing means for landing a subsequent casing hanger on a casing string, whereby the subsequent casing hanger and casing string may pass through the reduced diameter riser. The method may further comprise running a second string of casing through the reduced diameter riser and landing its casing hanger in the bore of the 13xe2x85x9cxe2x80x3 casing hanger.
Another embodiment of the present invention is directed to a method of running casing from an offshore vessel to a subsea wellhead comprising the steps of providing a lower marine riser package connector on the end of the reduced diameter riser to connect the riser to a lower marine riser mandrel on a blowout preventer stack; disconnecting the lower marine riser package connector from the lower marine riser mandrel; repositioning the riser over a secondary support mandrel on the blowout preventer stack; connecting the lower marine riser package connector to the secondary support mandrel; lowering a casing string outside the riser through the blowout preventer stack and into the well; landing a casing hanger for the casing string in a subsea wellhead housing, the casing hanger having an internal landing means in the bore of the hanger; releasing the lower marine riser package connector from the secondary support mandrel and reconnecting the lower marine riser package connector to the lower marine riser package mandrel on the blowout preventer; lowering a subsequent casing string through the riser and into the well; and landing the casing hanger for a subsequent casing string on the internal landing means of the previous hanger.